A series of alleged terrorist attacks shocked the U.S. today as Washington, D.C. and New York City came to a standstill following the hijacking and crashing of four passenger planes. Damage, death, and injury estimates stemming from today's terrorist attacks are still unavailable.

(4:50 p.m. ET) A series of alleged terrorist attacks shocked the U.S. today as Washington, D.C. and New York City came to a standstill following the hijacking and crashing of four passenger planes. The U.S. government, meanwhile, is already beginning an investigation into what is believed to be a coordinated terrorist attack. No group has yet claimed responsibility for the crashes and several have disavowed involvement.

Two American Airlines and two United Airlines flights carrying a total of 266 passengers, flight attendants, and pilots, were allegedly hijacked and crashed by terrorists. One of each airlines' planes crashed into New York's World Trade Center's twin towers, causing both buildings to collapse.

The second United flight is said to have crashed in western Pennsylvania, while the second American flight crashed into the Pentagon, the U.S. military's headquarters, where 24,000 work. The hijackers appear to have had their own pilots, one sign believed to point to a tightly orchestrated plan. All U.S. air travel has since stopped in an unprecedented shutdown.

No estimates of damage or loss of life are currently available, as it is unclear how many of the more than 150,000 who reportedly pass through the World Trade Center daily might have been affected by the crashes and the destruction of the buildings. Recovery efforts in Manhattan, meanwhile, have been slow in coming as they have been complicated by fire, debris, and crowding as people move north and rescue staff works to get into the city.

There were also injuries and fatalities at the Pentagon, say media accounts, though earlier reports of bombings at the State Department and Capitol -- many government offices in and around the D.C. area have been shut down and the museums and monuments are closed -- appear to be false.

The Capitol and White House have both been evacuated. U.S. Rep. Jim Moran [D-Va.] has said another plane, perhaps heading toward the Capitol, may have been intercepted.

First Lady Laura Bush and members of Congress have reportedly been moved to secure locations. The president reportedly will meet and discuss a response with defense and security leaders from another undisclosed location. He was in Florida but most recently stopped at a Louisiana Air Force base.

Work in the Pentagon's command center, meanwhile, reportedly continues as the U.S. attempts to identify the perpetrators. Among the suspects are terrorists associated with Saudi Arabian Osama bin Laden, one of the FBI's most wanted who is understood to lead one of the select few terrorist forces that might be able to execute such an assault.

Securities markets were closed today and will remain closed on Wednesday. They will open, according to a statement from SEC Chairman Harvey Pitt, "as soon as it is practicable to do so." Aid organizations and foreign governments, meanwhile, are already pledging their support and assistance. The Red Cross is seeking blood donors to help its relief efforts.

Fool community members are talking about the tragedy on the Current Events discussion board.